Diaphragm pumps with two diaphragms are already known from U.S. Pat. No. 4,049,366 (which is equivalent to German Auslegeschrift No. 25 02 556), wherein one working diaphragm closes the pump chamber, and on the side thereof opposite this pump chamber there is a further chamber closed by an additional diaphragm. However, not only the working diaphragm but preferably both diaphragms in that reference are approximately flat and level in the undeformed condition. The chamber situated between the working diaphragm and additional diaphragm takes the form of a damping space in which prevails a pressure reduced in comparison to the admission pressure of the pump, in such a way as to constantly draw the working diaphragm into a deflected position directed away from the pump chamber. Through the lower pressure acting on the side facing the crank-driving mechanism of the working diaphragm, one can ensure that the working diaphragm is constantly drawn into a deflected position directed away from the pump chamber. By this means the tendency the working diaphragm has to oscillate can be reduced to a considerable extent, thereby increasing the service life of the working diaphragm.
However, such diaphragm pumps, which have proved to be advantageous and successful in many respects, also have disadvantages: when working with very valuable gases, for instance costly inert gases, as the pumped medium works and the working diaphragm becomes loose, the valuable pumped gases quite often become contaminated because of an after-flow which can then occur from the damping space (i.e., the interspace between the diaphragms).
Contamination of the pumped gas is also particularly disadvantageous in swing connecting-rod pumps, because U packing rings or face seals are used there which have a leakage flow during operation and therefore do not permit the pump chamber to be fully sealed with respect to the crankcase zone. In principle, the interspace between the working diaphragm and the additional diaphragm (diaphragm pump), hereinafter referred to as diaphragm interspace, or between the U packing ring and the additional diaphragm (swing connecting-rod pump), hereinafter referred to as U packing ring interspace, could be substantially evacuated and the danger of contamination of the valuable pumped media could thereby be greatly reduced. However, this would involve the drawback that the freely movable, annular zone of the additional diaphragm deflects in the direction of the working diaphragm/U packing ring, and might even be exposed to alternating flexural load. In order to avoid unnecessary wear, measures would then also have to be taken to ensure that unwanted contact is not established between the additional diaphragm and working diaphragm (diaphragm pump) or between the additional diaphragm and the upper part of the connecting rod (swing connecting-rod pump).